The present invention relates to an arrangement for separating transmission and reception through an antenna in a transceiver. Such an arrangement can be employed to couple a transmitter/receiver of a radio apparatus to a common antenna in a mobile network based on TDMA, employing both FDD and TDD.
The great majority of the new mobile networks, and particularly those to be used in the future, are completely digital, and the network can be used for transferring various information. A mobile phone acting as a terminal of the network often employs the one and the same antenna for both transmission and reception. In such instances, various arrangements have to be adopted to ensure that a transmission will not enter the receiver and that a received transmission will not enter the transmitter.
In digital networks the TDM/TDMA principle is used, whereby transmission and reception are performed in different time slots. In such networks, if transmission and reception take place in different frequencies, a unit resembling the duplex filter used in analog networks can be used. Alternatively, if the transmission and reception frequencies are the same, an antenna switch (RF switch) can be used. In digital networks the FDM/FDMA principle is also used, whereby transmission and reception are accomplished in different frequency ranges, these being separated by a transition band. In such instances, a duplex filter can be used for signal separation.
Since the separation unit is the largest individual RF component of the terminal in items of volume and often also in mass, reducing its size, surface area and mass is desirable, in order to reduce the size of the mobile phone.
The function of a switch in an antenna switch structure is, while transmitting a signal, to couple the transmission port to the antenna while providing as low impedance and low loss signal path from the transmission port to the antenna port as possible. When transmitting a signal, the switch should appear to the receiver port as high impedance as possible, whereby the disturbance and spurious response signals are attenuated as efficiently as possible. When receiving a signal, the operation of the switch is reversed. The switch is then expected to couple the antenna port to the receiver port and offer as low impedance and low loss signal path as possible, and also, to attenuate signals towards the transmission port as efficiently as possible.
In a digital mobile phone employing Frequency Division Duplex, FDD, filters are needed, in addition to the RF switch, to separate reception and transmission and also to provide a sufficient selectivity and protection for the low noise preamplifier in the input of the receiver. At the output of the transmitter, harmonics of the output frequency must be attenuated, as well as other spurious transmissions such as mirror frequencies. In addition, there is often a need to filter noise generated by the transmitter chain to the receiver band. Similarly, attenuation of the frequencies below the transmission band has to be carried out with a separate filter. In a system employing time duplex, such as DECT, it is necessary to ensure by various measures, in addition to those described above, that when transmitting a signal, the spurious transmissions generated by the receiver towards the antenna are sufficiently attenuated.
FIG. 1 presents a known switch arrangement in which an antenna A is coupled with a logic-controlled RF switch K to the input of the receiver branch and to the output of the transmitter branch of a mobile phone. The antenna switch arrangement also includes filters S.sub.1 and S.sub.2. When transmitting a signal, control logic L controls the switch K into position T. The filter S.sub.2 arranged on the transmission branch attenuates the disturbance signals external to the transmission band, such as the harmonics of the output frequency and other distortions, leakage signals of oscillators, and other spurious transmissions, and filters noise external to the transmission band specifically in the reception band. When receiving a signal, the control logic controls the switch into position R. The filter S.sub.1 on the reception side attenuates from a signal passing through antenna A disturbance signals external to the reception band and protects the sensitive low noise preamplifier LNA against strong transmitter signals. This filter S.sub.1 also attenuates spurious transmissions outside the reception band propagating towards the antenna A in the other direction from the receiver RX.
U.S. Pat. No. 5,054,114 discloses this type of structure. Therein, a switch is produced with two PIN diodes. The transmitter is disposed in the antenna port with a wide band impedance adaptation circuit, and the isolation of the receiver is realized with a network which makes the zero impedance on the receiver side, produced when the PIN diode therein is conductive, appear in the antenna port as a high impedance. Here, the adaptation circuit and the network together form a bandpass filter implemented with discrete components or strip lines. No resonators are used.
The unit generally used in FDMA mobile phones for separating signals of different frequencies to be received and transmitted is a duplex filter which is also appropriate for a TDMA mobile phone. An advantage thereof is the single component of the structure when the discrete filters of the receiver and the transmitter are integrated into one. The duplex filters usually have good isolation and attenuation properties.
FIG. 2 presents a known arrangement based on duplex filter DPLX, in which a duplex filter is composed of two, typically passband filters S'.sub.1 and S'.sub.2. The transmission filter S'.sub.2 in the reception band appears to the antenna as a very high impedance and the reception filter S'.sub.1 has been adapted or matched to appear as being the magnitude of the antenna impedance. Likewise, due to the coupling together the receiver filter S'.sub.1 appears in the transmission band to the antenna as a very high impedance and the transmission filter S'.sub.2 is adapted or matched to the antenna impedance. The other functions of the filters S'.sub.1 and S'.sub.2 are the same as of the filters S.sub.1 and S.sub.2 of the apparatus presented in FIG. 1.
In the transmission band, the receiver filter S'.sub.1 therefore appears to a signal at transmission frequency as a very high impedance, attenuating to an acceptable level the signal which enters the receiver. The transmission filter S'.sub.2 functions likewise in the case of a reception frequency signal. The stop band attenuation requirement from the transmitter to the receiver in a digital TDMA mobile phone is of the order of 30 dB.
It is known in the state of art to combine an antenna switch and a filter within the one and the same structure. U.S. Pat. No. 5,023,935 discloses a design in which two adjacent quarter wave transmission lines serve as a filter, one end of one of said lines being connected to an antenna. The other transmission line has at one end been connected to the receiver and this end can be short circuited with a PIN diode. The other end has been connected to the transmitter through a reverse PIN diode. Using diodes and a transmission line, sufficient isolation is achieved, and, using parallel transmission lines connected electromagnetically together, a certain amount of filtering is obtained. However, with the design no high powers are achievable and the filtering properties are modest. A further drawback of the structure is great size. Therein, transmission lines being quarter wave in length are used, the length of which at 880 MHz frequency will be about 8.5 cm so that when the other switching elements are added the overall size will readily be about 20 mm.times.100 mm, which is far too much for a mobile phone.